There are basically two types of processes which can be used to sense the concentration of specific molecules in a liquid using a BAW quartz sensor. The two types of processes may be called by the generic names, active and passive. The active method is more descriptively and commonly called the oscillator method. In this method the BAW quartz sensor is part of an oscillator circuit. It is connected between the output and input of the oscillator amplifier and thereby provides positive feedback that causes oscillation of the circuit. The resonant frequency of the circuit is measured by an electronic counter. The quartz sensor is itself active in the sense that it is continuously controlling the frequency of oscillation of the circuit. The oscillator method is inadequate when used to sense molecules in a liquid.
The acoustic reflection process is a passive method in which the BAW quartz sensor is connected externally to an instrument which applies voltages, which typically vary sinusoidally with time, across the terminals of the sensor. Signal voltages are measured at various frequencies of the applied voltage. The quartz sensor does not determine the frequency at which measurements are made and in that sense the sensor itself is passive. The acoustic reflection process does not have the disadvantages of the oscillator method. Therefore, this invention displaces the oscillator method.
The oscillator method has at least three limitations when used to sense molecules in a liquid. Only one electrical quantity is measured and so the characterization of the sensor is incomplete. The measured quantity is a frequency which is ideally the series resonant frequency, defined as the lower of the two frequencies for which the phase of impedance of the sensor is zero. However, rarely is the measured frequency the same as the series resonant frequency due to unknown phase shifts elsewhere in the oscillator circuit; this is the second limitation. Thirdly, the oscillator method can be used only when the sensor is in liquids of low viscosity.
The acoustic reflection process completely characterizes a bulk acoustic wave quartz sensor. This is achieved by making measurements over the complete frequency spectrum of the resonant region of the quartz sensor. The process can be used with the sensor in a liquid of any viscosity.